The present invention relates to a lens retaining mount to retain a lens in a frame.
One of the known lens retaining mounts is disclosed in Japanese Laid-Open Patent Application No. 138606/1982 and is shown in FIG. 9. As shown, a lens 101 is retained in a Frame 102 which forms inner stepped portion 103 to receive the lens 101. The lens 101 is retained by a retainer ring 104 through an elastic member 105 which engages with outer periphery of the lens 101 and is inserted between the lens 101 and the retainer ring 104. Radial and axial thermal expansion and contraction is accomodated by the elasticity of the elastic member.
In this case, radial expansion of the lens 101 is absorbed by plastic deformation of the elastic member 105. When the lens 101 is shallow plastic lens, the elastic member 105 must be sufficiently soft to allow strain free expansion. However, as the radial positioning of the lens 101 is determined by the thickness uniformity of the elastic member, the elastic member should not be very soft.
Thus, elasticity of the elastic member 105 must be determined in narrow range. Even in such case accuracy of radial positioning of the lens 101 to the frame 102 can not be sufficiently high.
Some conventional lenses are retained in the frame by adhesive material. One of the prior art is disclosed in Japanese Laid Open Patent Application No. 1958111/1983 and is shown in FIG. 11. As shown, a lens 101 is accomodated in a lens receiving space 106 of a frame 102 and is stopped by a stepped portion 103 formed in the frame 102 to determine axial positioning of the lens 101. Between the outer periphery of the lens 101 and the inner periphery of the space 106, adhesive layer 107 is formed to retain the lens 101 in the frame 102.
To form the adhesive layer 107, at first the lens 101 is placed in the receiving space 106 and an injection needle or the like is used to fill clearance between the lens 101 and inner wall of the space 106 with suitable adhesive 107. Other method is that the adhesive is uniformly applied to the periphery of rotating lens 101 and the lens 101 is inserted in the frame to contact with the stepped portion 103.
However, some problems are caused by such prior art.
(1) As the lens 101 is pulled radially when the adhesive 107 is cured and contracted, so that interval strain is produced in the lens and decrease lens characteristics. PA0 (2) As the radial positioning of the lens 101 relative to the frame 102 must be performed before the curing of the adhesive material, non-uniform applications of adhesive or non-uniform clearance between the lens periphery and the inner wall of the frame 102, results in non-uniform thickness of the adhesive material so that when the adhesive is cured the lens apt to displace the lens position towards one radial direction. That means, radial positioning of the lens is not accurate. PA0 (3) As the adhesive layer 107 between the lens periphery and the inner wall of the frame 102 is relatively thin and relatively hard, radial thermal expansion between the lens 101 and the frame 102 can not be sufficiently absorbed by the adhesive layer 107, so that internal strains of the lens 101 causes disadvantageous effect to the lens.